Hollow, box-section, frame members are frequently required in many applications. These frame members are normally substantially rectangular in cross-section but have smoothed corners as sharp corners are potential weak spots. Furthermore, in many applications, it is often required that these frame members are curved in their length. The curved region of the frame member is often the weakest area of the frame member. This weakness results from stresses incurred while bending the frame member into its curved shape. However, in many applications, it is required that the frame member is sufficiently strong throughout its length.
The applicant in U.S. Pat. No. 4,567,743, which issued Feb. 4, 1986, describes a method in which a tubular work piece or blank may be formed into a hollow, box-section, frame member. In this method, a tubular blank is first bent into a required curved shape. Then, the curved blank is placed in a preforming die to deform the sidewalls of the blank. The sidewalls are then inwardly recessed and concavely curved in areas corresponding to the areas that will form proposed planar sidewalls in the final frame member. The circumference of the deformed blank is desirably no less than about 95% of the circumference of the final product. This allows the deformed blank to be placed in a final die, which has a cavity corresponding to the desired cross-sectional shape of the final frame member, and the die to be closed without pinching the wall of the blank. The blank is then expanded by an internal fluid pressure which exceeds the yield limit of its sidewall. The sidewall thus expands outwardly to conform to the interior of the final die cavity. The method thus provides a convenient method of forming hollow, box-section, frame members.
The applicant, in a U.S. application entitled "Method of Forming Box-like Frame Members", executed on Apr. 29, 1987, discloses an improvement to this basic method. As noted above, the preforming step in the original method is required to prevent the incidence of pinching of the blank within the final die. This pinching results from frictional drag exerted on the blank by the surface of the die cavity. In this latter application, the inventor has found that the frictional drag can be overcome by pressurizing the blank with an internal fluid pressure less than the yield limit of the sidewall of the blank before closing the die sections. As the die sections are closed, the internal pressure causes the sidewall of the blank to bend evenly into the corners of the final die. The sidewall of the blank thus slips over the die cavity surface and avoids the pinching problem. This improvement of the original method avoids the need for a preforming die.
Although the above-described methods solved many of the problems associated with the manufacture of hollow, box-section frame members, the frame members thus produced are not provided with localized reinforcing to prevent weak spots. Accordingly, it is an object of this invention, to provide a method of forming hollow, box-section, frame members which include localized reinforcing.